The shower symptoms suggest the mixer may be faulty. I had a faulty shower mixer that allowed cross-flow between hot and cold, even when in the off position, so as soon as I turn off the hot tap, cold water backfed into the boiler. Don't immediately see how that could cause your boiler symptoms but worth checking out - once your system is not working as designed lots of weird things can happen! Fixed by putting non-return valves in the the pipes to the shower (replacing the shower will be done sometime but is a big job as the pipes in the wall are non-standard).

Could be a faulty flow rate sensor . Lowish flow = very hot water , sensor then tells boiler tap/shower is off ( when it isnt ) and turns off heat. Water still runs but goes cold.
See if you can recreate this by popping the cover and running a hot tap in the bathroom.
The boiler will fire , and you will see flames, get your assistant to slowly turn down the flow rate. Shout when flames go out , if tap is still running then you have a problem somewhere.

thanks all single track mind sounds the mot likely as the shower is only a year old. However I am inclined to agree with dales rider and blame a rift in the space time continuum.
thanks all will investigate